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| Bubbling methane melted a hole in the ice of this otherwise frozen lake
in the Brooks Range, Alaska, in April 2011. Credit: Katey Walter Anthony |
By Laura Naranjo
As people watch the decline of Arctic sea ice, the most obvious sign
of climate warming in that region, scientists are noting other signs of
change, like methane seeping out of the ground as permafrost thaws and
glaciers melt across the Arctic. Scientists suspected these methane
seeps existed, but no one had measured how much methane was escaping --
until recently.
After working for nearly 10 years on the ground studying Siberian
lakes, Katey Walter Anthony, an aquatic ecosystem ecologist at the
University of Alaska, was flying over the Alaska tundra in 2008 when she
spotted something odd in the lakes there. She said, "There were large
open areas in some lakes, which at that time of year should have been
frozen solid. When we got to these sites on the ground, we saw large
plumes of bubbling gas -- it looked like these parts of the lake were
boiling."
These upwellings were plumes of methane, seeping out of the ground
and up through the water. The convection associated with the bubbling
prevented the ice from freezing. Where does this methane come from? And
does its escape mean more warming in the Arctic?
From whence the gas?
Methane is trapped in and beneath the permafrost overlying the
Arctic's sedimentary basins, and is common in the organic material
deposited by glaciers, and in marshy lakes and ponds. The frozen soil
acts like a bathtub, holding water in the lake basins and preventing
methane beneath the permafrost from percolating to the surface. When the
permafrost thaws beneath lakes, gas-permeable chimneys open up, and the
methane seeps out.
Walter Anthony and her team started to investigate these large
methane seeps, which are thought to occur all across the Arctic. Having
surveyed Alaska and Greenland using airplanes and field expeditions on
the ground, they discovered more than 150,000 seeps. There are likely
many more across the vast reaches of Arctic Russia and Canada, and the
team hopes to use remote sensing to confirm this.
During ground surveys, they examined the chemical and isotope
composition of the bubbling methane to determine where it was coming
from. In many of the smaller bubbling seeps methane was newer, formed
when plants and other organic material decayed in the lakes. However,
they found that the largest seeps were out-gassing fossil methane from
ancient sources, such as natural gas and coal beds. Much of the seeping
geologic methane had been trapped underground for tens of thousands of
years, meaning that permafrost was thawing to such an extent that it was
finally releasing those long-stored gases.
Worry about methane?
Seeping methane is worrisome because it is a potent greenhouse gas.
Melanie Engram, a researcher at the University of Alaska and a colleague
of Walter Anthony’s said, "Methane is twenty-five to twenty-eight times
more effective at retaining heat as carbon dioxide." Engram is one of
the researchers working with Walter Anthony to figure out how to measure
the amount of methane now seeping out. When scientists model the
effects of greenhouse gases, they need to account for as many sources of
a gas as possible, now including these seeps. "Currently there is no
quantification for these lakes in the methane budget," Engram said. "One
of the most exciting aspects of this project is investigating the use
of synthetic aperture radar (SAR) satellite imagery, provided by NASA
through the Alaska Satellite Facility, to quantify methane bubbles
trapped by lake ice." SAR remote sensing, which can image through clouds
and at night, is a tool well-suited for monitoring northern landscapes
during dark Arctic winters. Satellite remote sensing is valuable for
providing images of remote Arctic regions that would otherwise be too
logistically difficult or expensive to observe.
And as permafrost warms and glaciers recede across the Arctic, the
frozen cap locking methane underground will continue to thaw. Scientists
are still trying to understand the extent of seeping methane. If
thawing continues, Walter Anthony estimates that more than ten times the
amount of methane currently in the atmosphere may bubble up out of the
lakes. More methane could fuel the feedback loop that further warms the
Arctic and the global atmosphere.
Read more in The Alaska Dispatch (original article in the
Icelights blog, produced by the National Snow and Ice Data Center)
